Stanislav Brebera spent much of his life developing Semtex, the best plastic explosive in the world. It feels like Play Dough, has no smell, and was designed in 1966 to clear land-mines and improve industrial safety. It is also undetectable by dogs and airport security devices, and after it left Mr. Brebera's laboratory in 1968, Semtex became the favored weapon of international terrorists from Libya to Northern Ireland.

Since Sept. 11, the Czech Republic and its new NATO allies have become increasingly nervous about the continued production and sale of Brebera's fatal concoction.

Over the past two decades, terrorists have employed Semtex in several deadly attacks, including the 1988 explosion of Pan Am flight 103 over Lockerbie, Scotland, and the 1998 bombing of the US Embassy in Nairobi, Kenya. And no one has found a reliable way to combat it.

The Czech government recently decided to take control of Explosia, the factory that produces the explosive, in hopes of stopping the misuse of Semtex. The government will buy the plant from its parent company, Synthesia Pardubice.

Yet, some analysts doubt the move will have much effect. "I don't think state ownership is the answer," says Jonathan Stein, a Prague-based analyst for the Economist Intelligence Unit. "That won't control what happens to it after it leaves the factory."

Named after Semtin, the village in East Bohemia where Brebera invented it, this extraordinarily stable compound of RDX (Cyclonite) and PETN (Penaerythrite Tetranitrate) slips through airport security scans as easily as a pair of nylons. According to the FBI, Semtex has an indefinite shelf life and is far stronger than traditional explosives such as TNT. It is also easily available on the black market.

Semtex became infamous when just 12 ounces of the substance, molded inside a Toshiba cassette recorder, blasted Pan Am flight 103 out of the sky above Lockerbie, Scotland, in December 1988, killing 270 people. A year later, after the Czech Communist regime was toppled, the new president, Vaclav Havel, revealed that the Czechs had exported 900 tons of Semtex to Col. Moammar Qaddafi's Libya and another 1,000 tons to other unstable states, such as Syria, North Korea, Iraq, and Iran. Some experts now put worldwide stockpiles of Semtex at 40,000 tons.

Brebera says that with so much Semtex already in the hands of terrorists, and similar explosives being produced in other countries, the Czech Republic can no longer control it. "Semtex is no worse an explosive than any other," he says, defensive at the sight of accusatory headlines in Western newspapers. "The American explosive C4 is just as invisible to airport X-rays, but they don't like to mention that."

After the Lockerbie tragedy, Brebera added metal components and a distinct odor to make Semtex easier to detect. But that did not stop terrorists from using it to bomb the US Embassy in Nairobi, Kenya, in 1998, or prevent the IRA, which received about 10 tons of Semtex from Libya, from continuing its attacks.

Although the Czech government officially monitors all Semtex sales, there has been a string of cases recently in which cash-strapped Czech companies were discovered illegally selling weapons to old trade partners in Yugoslavia, Iraq, and North Korea.

Semtex also is often stolen from industrial sites. Last year, a group of smugglers was caught transporting through Poland enough Semtex to make 40 powerful bombs. In fact, experts say it is now easier than ever to obtain the explosive.

Last December, a US passenger plane narrowly escaped another Czech invention when Richard Reid was caught trying to board a plane with a homemade explosive in his shoes. According to CNN, he obtained the recipe from an unnamed "Czechoslovak." Brebera says he produced similar concoctions as a boy in the 1930s, and it is fortunate that Reid didn't have Semtex, because if he had, his attempt might not have been detected.

"When I was 10, I made little bombs out of keys, nails and the dust of matches, and later I made white powder explosives," he says. "You just mixed a nasty weed-killer, which is thankfully now banned, with powdered sugar. It made a wonderful explosive, but it was extremely unstable."

As a youth, Brebera gave up his other obsession - Czech and Russian literature - to attend the Czech High Technical Academy in Prague to study chemistry. After graduation in 1950, he entered mandatory military service. His superiors noticed his talents and sent him to the Military Technical Institute, where the Soviet bloc's race to develop the best plastic explosive was already under way.

"At first I was unhappy about it," Brebera says. "I am not a military man, and all the regulations got on my nerves. But I really did love the chemistry, and eventually I lost myself in it."

The trouble began in 1966, when Brebera received orders to produce a plastic explosive for the North Vietnamese as an answer to American C4, which was being used in the Vietnam War for heavy artillery. "It was not supposed to be that much different from C4, but my secret was in the styrene-butadiene rubber binder, which had a very positive effect on the consistency," Brebera says. The result was today's Semtex.

Brebera lost all control over his invention when the Soviet Army invaded Czechoslovakia in 1968, crushing a reform movement that Brebera and his wife had joined. He was stripped of his oversight powers and demoted, while his son was barred from university studies.

The Vietnam War ended after only 14 tons of Semtex had been delivered, and the excess was shipped to Soviet allies to buy political favors, leaving a time bomb of massive stockpiles around the world. To Brebera's credit, he has not improved Semtex since its blatant misuse began in the 1960s, and it is still by far the best plastic explosive in the world.

Brebera is now a modest, graying man, who owns little, not even a car. He lectures at Pardubice University to supplement a meager pension.

"It makes me angry that Semtex fell into the wrong hands," he says. "Now, the world blames my country and me, but I could not stop it. I know now that if you are going to invent, you run the risk that someone will use your creation for something you didn't intend."